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Thilker, David A.,Bianchi, Luciana,Schiminovich, David,Gil de Paz, Armando,Seibert, Mark,Madore, Barry F.,Wyder, Ted,Rich, R. Michael,Yi, Sukyoung,Barlow, Tom,Conrow, Tim,Forster, Karl,Friedman, Peter IOP Publishing 2010 ASTROPHYSICAL JOURNAL LETTERS - Vol.714 No.1
<P>We have discovered recent star formation in the outermost portion ((1-4) x R(25)) of the nearby lenticular (S0) galaxy NGC 404 using Galaxy Evolution Explorer UV imaging. FUV-bright sources are strongly concentrated within the galaxy's Hi ring (formed by a merger event according to del Rio et al.), even though the average gas density is dynamically subcritical. Archival Hubble Space Telescope imaging reveals resolved upper main-sequence stars and conclusively demonstrates that the UV light originates from recent star formation activity. We present FUV, NUV radial surface brightness profiles, and integrated magnitudes for NGC 404. Within the ring, the average star formation rate (SFR) surface density (Sigma(SFR)) is similar to 2.2 x 10(-5) M(circle dot) yr(-1) kpc(-2). Of the total FUV flux, 70% comes from the H I ring which is forming stars at a rate of 2.5 x 10(-3) M(circle dot) yr(-1). The gas consumption timescale, assuming a constant SFR and no gas recycling, is several times the age of the universe. In the context of the UV-optical galaxy color-magnitude diagram, the presence of the star-forming Hi ring places NGC 404 in the green valley separating the red and blue sequences. The rejuvenated lenticular galaxy has experienced a merger-induced, disk-building excursion away from the red sequence toward bluer colors, where it may evolve quiescently or (if appropriately triggered) experience a burst capable of placing it on the blue/star-forming sequence for up to similar to 1 Gyr. The green valley galaxy population is heterogeneous, with most systems transitioning from blue to red but others evolving in the opposite sense due to acquisition of fresh gas through various channels.</P>
THE THICK DISK IN THE GALAXY NGC 4244 FROM S<sup>4</sup>G IMAGING
Comeró,n, Sé,bastien,Knapen, Johan H.,Sheth, Kartik,Regan, Michael W.,Hinz, Joannah L.,Gil de Paz, Armando,Mené,ndez-Delmestre, Karí,n,Muñ,oz-Mateos, Juan-Carlos,Seibert, IOP Publishing 2011 The Astrophysical journal Vol.729 No.1
<P>If thick disks are ubiquitous and a natural product of disk galaxy formation and/or evolution processes, all undisturbed galaxies that have evolved during a significant fraction of a Hubble time should have a thick disk. The late-type spiral galaxy NGC 4244 has been reported as the only nearby edge-on galaxy without a confirmed thick disk. Using data from the Spitzer Survey of Stellar Structure in Galaxies (S(4)G) we have identified signs of two disk components in this galaxy. The asymmetries between the light profiles on both sides of the mid-plane of NGC 4244 can be explained by a combination of the galaxy not being perfectly edge-on and a certain degree of opacity of the thin disk. We argue that the subtlety of the thick disk is a consequence of either a limited secular evolution in NGC 4244, a small fraction of stellar material in the fragments which built the galaxy, or a high amount of gaseous accretion after the formation of the galaxy.</P>
Comeró,n, Sé,bastien,Elmegreen, Bruce G.,Knapen, Johan H.,Sheth, Kartik,Hinz, Joannah L.,Regan, Michael W.,Gil de Paz, Armando,Muñ,oz-Mateos, Juan-Carlos,Mené,ndez-Delmestre, K IOP Publishing 2011 ASTROPHYSICAL JOURNAL LETTERS - Vol.738 No.2
<P>NGC 4013 is a nearby Sb edge-on galaxy known for its 'prodigious' Hi warp and its 'giant' tidal stream. Previous work on this unusual object shows that it cannot be fitted satisfactorily by a canonical thin+thick disk structure. We have produced a new decomposition of NGC 4013, considering three stellar flattened components (thin+thick disk plus an extra and more extended component) and one gaseous disk. All four components are considered to be gravitationally coupled and isothermal. To do so, we have used the 3.6 mu m images from the Spitzer Survey of Stellar Structure in Galaxies. We find evidence for NGC 4013 indeed having a thin and a thick disk and an extra flattened component. This smooth and extended component (scale height z(EC) similar to 3 kpc) could be interpreted as a thick disk or as a squashed ellipsoidal halo and contains similar to 20% of the total mass of all three stellar components. We argue it is unlikely to be related to the ongoing merger or due to the off-plane stars from a warp in the other two disk components. Instead, we favor a scenario in which the thick disk and the extended component were formed in a two-stage process, in which an initially thick disk has been dynamically heated by a merger soon enough in the galaxy history to have a new thick disk formed within it.</P>